Formation process of the trace fossil Phymatoderma granulata in the Lower Jurassic black shale (Posidonia Shale, southern Germany) and its paleoecological implications

摘要

The ichnogenus Phymatoderma is a subhorizontal branching burrow system consisting of radiating tunnels filled with fecal pellets. This ichnogenus has been interpreted as a product of a deposit-feeding animal, but the question of whether the Phymatoderma-producer was a subsurface deposit feeder or a surface deposit feeder is still a topic of controversy. Herein I present evidence, based on carbon-isotope analyses, for the formation of the trace fossil Phymatoderma granulata occurring in the lower Toarcian black shale in southern Germany. Carbon-isotope ratios of organic carbon (δ ~(13)C _(org)) in the pelletal infill of P. granulata, the surrounding black shale, and the overlying gray mudstone are -26.64±0.42 (2σ) ‰, -28.49±0.44 (2σ) ‰, and -26.27±0.13 (2σ) ‰, respectively. The difference between the pelletal infill and overlying mudstone in terms of δ ~(13)C _(org) is much smaller than that between the fillings and black shale; therefore, these data clearly indicate that the Phymatoderma-producer ingested the surface sediments and subsequently excreted fecal pellets into the subsurface sediments. Such a surface deposit-feeding style would be an especially effective means of absorbing nutrients, because surface sediments contain much fresh organic material, whereas organic matter in subsurface deposits consists mostly of refractory material that is poorly utilized by most marine benthos.